1. Field of the Invention
The present invention relates to an audio broadcast receiver for receiving AM, FM, and other analog audio broadcasts and more particularly to an audio broadcast receiver in which automatic broadcasting-station selection is performed.
2. Description of the Related Art
Automatic broadcasting-station selection in audio broadcast receivers is a technology in which automatic searching for audio broadcast frequencies with good reception is performed, an audio broadcast is received at the frequency found, and the audio broadcast is reproduced.
As such an automatic broadcasting-station selecting technology, there is a known technology in which, while receiving frequencies are changed, a frequency at which an electric field strength equal to or more than a predetermined value can be obtained and the intermediate frequency converted from the receiving signal is in a prescribed frequency band is searched for, the searched-for frequency is fixed as a receiving frequency, and a broadcast at the receiving frequency is received and reproduced to users (for example, Japanese Unexamined Patent Application Publication No. 9-46184).
According to the above conventional technology in which automatic broadcasting-station selection is performed in accordance with the received electric field strength and the intermediate frequency, when spurious noise (unwanted waves) exists, there are cases in which audio broadcasting stations to be correctly and well received cannot be selected.
Furthermore, in the case where IBOC (In-band On-channel) audio broadcasting is performed where digital broadcasting is carried out by using an analog broadcast frequency band, when an analog audio broadcast is automatically searched for, it becomes impossible to select an analog audio broadcast which can be correctly and well received because of the influence of digital broadcasting.
Here, in FIGS. 5A to 5D, the spectrum distribution in AM audio broadcasting of the IBOC method is shown. In IBOC AM audio broadcasting, there are full-digital broadcasting where only digital broadcasting is performed and half-digital broadcasting where digital broadcasting is multiplexed with analog audio broadcasting. The spectrum distribution of half-digital broadcasting is shown in FIG. 5A and the spectrum distribution of full-digital broadcasting is shown in FIG. 5B.
In the USA where IBOC AM audio broadcasting is performed, since broadcast frequencies are distributed among AM audio broadcasting stations so that the broadcast frequencies are multiples of 10 kHz, in audio broadcast receivers, it is necessary to search for analog audio broadcast that can be well received while receiving frequencies which are changed in units of 10 kHz.
Then, according to the above conventional technology in which automatic broadcasting-station selection is performed in accordance with the received electric field strength and the intermediate frequency, full-digital broadcast frequencies shown in FIG. 5B are also judged to be broadcast frequencies where audio broadcasts can be well received, and, as a result, although analog audio broadcasts are not contained within the broadcast frequencies, the broadcast frequencies are fixed and the receiver starts to reproduce them to users.
Furthermore, as understood from FIGS. 5A and 5B, since the spectrum of digital broadcasting in IBOC AM audio broadcasting is distributed beyond a 10 kHz range centered around the broadcasting frequency, although neighboring frequencies at intervals of 10 kHz and analog broadcasts that can be well received at those frequencies do not exist, those frequencies are judged to be broadcasting frequencies where audio broadcast can be well received, and the broadcasting frequencies are thus fixed and the receiver starts to reproduce them to users.